Method for non-contact stress evaluation of wafer gate dielectric reliability

ABSTRACT

An apparatus and method for evaluating the performance of a test dielectric material for use as a gate dielectric. The method comprises exposing a coated layer of the dielectric to a concentration of atomic hydrogen. The method may comprise (a) measuring an initial value of interface-state density in the test dielectric, (b) exposing the coated test dielectric to a concentration of atomic hydrogen in a remote plasma, and then (c) measuring a post-exposure value of interface-state density in the test dielectric. Steps (b) and (c) may be repeated with incrementally higher concentrations of atomic hydrogen to determine a rate of change in interface-state density value as a function of atomic hydrogen concentration, which may then be related to the projected charge-to-breakdown or time-to-breakdown of the test dielectric layer when the dielectric is used as the gate dielectric. The method may be conducted on a remote-plasma hydrogen exposure apparatus comprising, in series, a source of a mixture of molecular and atomic hydrogen gas; a particle remover adapted to remove energetic, charged particles; a light sink; a hydrogen recombination device; and a wafer exposure chamber.

This application is a divisional of U.S. patent application Ser. No.09/250,880, filed on Feb. 16, 1999, which is now U.S. Pat. No.6,326,732.

TECHNICAL FIELD

The present invention relates generally to evaluation of wafer gatedielectric reliability in semiconductor products and, more specifically,to an apparatus and method for gathering accelerated life stress datafor evaluation of wafer gate dielectric reliability.

BACKGROUND OF THE INVENTION

Thin gate dielectric reliability and integrity constitute one of themajor challenges and concerns for the development and manufacturing ofVLSI (very large scale integration) and ULSI (ultra large scaleintegration) semiconductor products. The development of reliable andhigh quality thin gate dielectrics requires a research- andtime-intensive effort to meet continuously evolving competitive demandsfor smaller device geometries and better performance and reliability. Asthe thickness of the gate dielectric continues to be reduced to meetindustry demands, continuous process improvements are necessary to meetyield and reliability criteria. The selection of optimum gate dielectricprocesses demands an intensive effort to collect life stress data on thewafers produced by the various processes.

It is currently known to collect life stress data using a chip-by-chipstress procedure in which each chip is contacted with a probe, one at atime, to measure the effects of lifetime-accelerated voltage andtemperature conditions. A wafer typically contains a plurality of chipsrepeatedly patterned across its surface, a chip being a basic integratedcircuit device unit. All chips on a wafer are initially tested atactual-use voltage conditions to establish the initial quality of gatedielectric. Then, voltage and temperature stresses are applied to eachchip for a certain amount of time ranging from a few minutes to severalhours or more. After the application of accelerated life stressconditions, the chips are tested again, and a significant increase ingate leakage from its initial value signifies dielectric breakdown.

Using advanced modeling, the stress data is then extrapolated to aprojection of reliability under actual-use conditions. This projectionis used to make comparisons between various process options. Because themanufacturing process does not permit time-consuming testing of eachchip of every wafer produced, random sampling with relatively shorttesting periods is required. Still, the chip-by-chip contacting systemis a labor- and time-intensive process and contributes significantly tothe overall development cost. An evaluation procedure is needed wherebythe quality of the dielectric on the whole wafer may be evaluated,especially an evaluation procedure that may be conducted before thefabrication of the electronic devices.

SUMMARY OF THE INVENTION

To meet this and other needs, and in view of its purposes, the presentinvention provides a method for evaluating the performance of a testdielectric material for use as a gate dielectric. The method comprisesexposing a coated layer of the dielectric to a concentration of atomichydrogen. The method also may comprise (a) measuring an initial value ofinterface-state density in the test dielectric, (b) exposing the coatedtest dielectric to a concentration of atomic hydrogen in a remoteplasma, and then (c) measuring a post-exposure value of interface-statedensity in the test dielectric. The method may further comprise (d)repeating steps (b) and (c) at least one more time with an incrementallyhigher concentration of atomic hydrogen each time steps (b) and (c) arerepeated; and (e) determining a rate of change in interface-statedensity value as a function of atomic hydrogen concentration. The methodmay then comprise relating the rate of change in interface-state densityvalue to projected charge-to-breakdown or time-to-breakdown of the testdielectric layer for use of the dielectric as the gate dielectric.

The method of the present invention may be conducted on a remote-plasmahydrogen exposure apparatus, comprising, in series, a source of amixture of molecular and atomic hydrogen gas; a particle remover adaptedto remove energetic, charged particles; a light sink; a hydrogenrecombination device; and a wafer exposure chamber. It is to beunderstood that both the foregoing general description and the followingdetailed description are exemplary, but are not restrictive, of theinvention.

BRIEF DESCRIPTION OF DRAWING

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawing. It is emphasizedthat, according to common practice, the various features of the drawingare not to scale. On the contrary, the dimensions of the variousfeatures are arbitrarily expanded or reduced for clarity. Included inthe drawing are the following figures:

FIG. 1 is a schematic illustration of an exemplary remote-plasmahydrogen exposure apparatus according to the present invention;

FIG. 2 is a flowchart of an exemplary method of the present invention;

FIG. 3 is a graph of injected charge density versus change ininterface-state density as measured by conventional contacting methodsusing photoinjection at 23° C. with an electric field of 4 MV/cm(positive gate) applied to the gate electrode;

FIG. 4 is a graph of hydrogen dose versus interface state buildup at thesubstrate/oxide interface; and

FIG. 5 is a graph of oxide thickness versus critical defect density atbreakdown.

DETAILED DESCRIPTION OF INVENTION

Referring now to the drawing, in which like reference numerals refer tolike elements throughout, FIG. 1 illustrates an exemplary remote-plasmahydrogen exposure apparatus 10. The apparatus as illustrated comprises,in series, four general parts: a gas inlet system 12, a gas conditioningsystem 14, a wafer exposure area 16, and optionally, a load-lock system18.

Gas inlet system 12 provides a source of a mixture of molecular andatomic hydrogen gas. The inlet system comprises a source of molecularhydrogen gas connected to inlet manifold 20 and an atomic hydrogenproduction unit 22 adapted to produce atomic hydrogen from the molecularhydrogen. Inlet manifold 20, as shown in FIG. 1, may comprise multipleinlet connections 21, so that the hydrogen may be connected to one inletconnection and other gases, such as argon or helium, may be connected tothe remaining inlet connections to facilitate mixing of the hydrogenwith other gases.

Atomic hydrogen production unit 22 may comprise, for example, a plasmasource such as a microwave discharge in a quartz tube, or a hydrogencracker such as a hot tungsten filament, both of which are known in theart. Gas flow may be controlled by microvalve 24.

Gas conditioning system 14 comprises a particle remover 30 adapted toremove energetic, charged particles; a light sink 32; and a hydrogenrecombination device 33. Particle remover 30 may be an electrostaticgrid across a drift tube adapted to collect ions of a particular chargeaccording to a voltage polarity on the grid. For instance, electrostaticgrid particle remover 30 may collect positive ions and confine negativeions to the gas inlet system, or vice versa, depending on the voltagepolarity on the grid.

Light sink 32 may be a transport tube, as shown in FIG. 1, havinginterior walls 34 and containing light baffles 36. Interior walls 34 andlight baffles 36 may be coated with a hydrogen recombination inhibitor,such as polytetrafluoroethylene (PTFE). The baffled transport tube lightsink 32 removes light by multiple reflection and absorption. The lightintensity decreases exponentially with the number of light baffles 36 inthe tube. The PTFE coating suppresses hydrogen recombination on thewalls of the transport tube, so that the amount of atomic hydrogen canbe maximized.

The step of conditioning the gas to remove energetic, charged particlesand light (specifically UV radiation) is necessary to avoiduncontrollable stress on the wafer. Both UV radiation and charged orenergetic particles are known to degrade gate oxides. Thus, thesedegradation mechanisms must be eliminated so that only stresses createdby atomic hydrogen exposure are present.

Hydrogen recombination device 33 may further comprise a metal-walledtube 40 having a variable length, such as the set of bellows shown inFIG. 1. The bellows comprise a fixed section 42 and a movable section 44connected by flexible sections 46. As movable section 44 traverses inthe direction as shown by arrow “A”, the overall length of tube 40 fromentry 48 to exit 50 increases. When movable section 44 traverses in thedirection opposite arrow “A”, the length of tube 40 decreases.

Recombination device 33 reduces and adjusts the atomic hydrogen densityin the gas mixture to a desired value. Atomic hydrogen efficientlyrecombines on metal walls, and so the degree of recombination can becontrolled simply by varying the length of tube 40. A set of bellowsenables significant variation of the tube length, thus providing gasratio control over several orders of magnitude.

The wafer exposure area 16 comprises a wafer exposure chamber 52.Recombination device 33 may further comprise a pre-exposure chamber 54in series with exposure chamber 52. An exposure valve 56 is placedbetween the two chambers 52, 54. A bypass 58 connecting chambers 52 and54 may be present to enable the pressure to be equalized between thechambers without opening exposure valve 56. Vacuum line 70 is connectedto a vacuum source (not shown) to keep the system operating undervacuum, typically maintaining an internal pressure of approximately 10⁻³Torr in apparatus 10.

A wafer 60 and a corresponding wafer holder 82 are shown in FIG. 1positioned in exposure chamber 52 with solid lines and positioned inload-lock chamber 65 with broken lines because the wafer and waferholder are not in both positions simultaneously. Bypass 58 is preferablymetal-walled and long enough that full recombination occurs before thegas reaches exposure chamber 52, so that wafer 60, as shown in FIG. 1with solid lines, is not exposed to atomic hydrogen while the pressureis being equalized. Bypass 58 further comprises a block valve 62 that isopened to equalize pressure, but remains closed during wafer exposure.

Inside exposure chamber 52, wafer 60 is mounted to wafer holder 82,which is in turn mounted to a heatable chuck 64. Wafer holder 82 furthercomprises a connector 84, which may be a male threaded coupling as shownin FIG. 1, adapted to mate to a connector 67, such as a female threadedcoupling, on the top of transport mechanism 68. Wafer holder 82 mayfurther comprise a shaped ridge 86 adapted to fit into a mating groove63 in chuck 64.

Load-lock system 18 is adapted to allow removal of wafer 60 and attachedwafer holder 82 from wafer exposure chamber 52 and reloading of anotherwafer without stopping the gas flow. Door 66 on load-lock chamber 65opens to enable wafer 60 and wafer holder 82, as shown in FIG. 1 withbroken lines, to be unloaded from or a new wafer and wafer holder to beloaded onto transport mechanism 68. For a wafer holder 82 such as isshown in FIG. 1, loading or unloading comprises screwing or unscrewingwafer holder connector 84 onto transport mechanism connector 67. Toprevent atmospheric air from leaking into load-lock chamber 65,transport mechanism 68 as shown in FIG. 1 includes a seal 69 wheretransport mechanism 68 pierces load-lock chamber 65. Other types oftransport mechanisms, such as a magnetic-based system, may also be usedthat may or may not require such a seal.

During the unloading operation, door 66, block valve 62, and exposurevalve 56 are closed and load-lock valve 72 is opened to enable transportmechanism 68 to extend along arrow “B” into exposure chamber 52 toprocure wafer 60, as shown with solid lines in FIG. 1. For a threadedwafer holder 82, such as is shown in FIG. 1, transport mechanism 68 istwisted to screw into wafer holder 82, and then transport mechanism 68is moved to slide ridge 86 out of groove 63. Once wafer holder 82 isfree from chuck 64, transport mechanism 68 may then be retracted in thedirection opposite arrow “B” to put wafer 60, as shown with broken linesin FIG. 1, in reach of door 66. Load-lock valve 72 is closed, door 66 isopened, the old wafer 60 is removed, and a new wafer is placed ontransport mechanism 68 in its place.

The loading procedure comprises a reverse procedure from the unloadingprocedure: door 66 is closed, load-lock valve 72 is opened, transportmechanism 68 is inserted in the direction of arrow “B”, and wafer holder82 is mounted on chuck 64. After wafer 60 and wafer holder 82, as shownwith solid lines in FIG. 1, are mounted on chuck 64, transport mechanism68 is retracted, load-lock valve 72 is closed, and block valve 62 isopened. Once the pressure is equalized between exposure chamber 52 andpre-exposure chamber 54, exposure valve 56 is opened and block valve 62is closed to begin exposure of the new wafer.

The ratio of atomic and molecular hydrogen in pre-exposure chamber 54and exposure chamber 52 may be measured using bolometric sensors 74 and74′, which may be silver-coated thermocouples. Atomic hydrogen has ahigh sticking and recombination coefficient on silver, and the heatproduced by such recombination is a direct measure of the atomichydrogen concentration. For starting up the apparatus, exposure valve 56is closed and block valve 62 is open so that the vacuum pulled throughvacuum line 70 can bring chambers 52 and 54 to equilibrium. Once thesurfaces of the sensor 74 have been conditioned and a stable, desiredratio of gases is reached in pre-exposure chamber 54, exposure valve 56is opened and block valve 62 is closed to begin wafer exposure.

A remote-plasma hydrogen exposure apparatus, such as apparatus 10described above, may be used to conduct a method of the presentinvention. The method evaluates the reliability and integrity of a gatedielectric on a sample wafer by exposing a coated layer of thedielectric to a concentration of atomic hydrogen. Several studies haveindicated that the release of hydrogen and its reactions are a majorsource for hot-electron degradation leading to dielectric breakdown. Ithas also been shown, by E. Cartier, D. A. Buchanan, and G. J. Dunn, in“Atomic Hydrogen-Induced Interface Degradation of Reoxidized-NitridedSilicon Dioxide on Silicon,” Appl. Phys. Lett. 64(7), pages 901-03 (Feb.14, 1994), that degradation of gate dielectrics by atomic hydrogen hascharacteristics similar to degradation induced by hot electronstressing.

Referring now to FIG. 2, there is shown a flowchart depicting anexemplary method in accordance with the present invention. The methodcomprises, at step 100, first measuring an initial value ofinterface-state density in the gate dielectric. Then, at step 110, thesample wafer is exposed to a concentration of atomic hydrogen in aremote plasma. This exposure in step 110 may include using remote-plasmahydrogen exposure apparatus 10 as shown in FIG. 1 and as describedabove. Next, in step 120, the post-exposure value of interface-statedensity in the gate dielectric is measured. At step 130, steps 110 and120 are repeated with an incrementally higher concentration of atomichydrogen each time the steps are repeated, until a satisfactorily highconcentration has been reached. At step 140, the rate of change ininterface-state density as a function of atomic hydrogen concentrationis determined. Finally, at step 150, the rate of change ininterface-state density is related to a projected dielectric lifetime.Specifically, step 150 may comprise relating the rate of change ininterface-state density to the charge-to-breakdown or time-to-breakdownof the dielectric.

Exposure of a wafer to atomic hydrogen induces a stress on the thin gatedielectric that is effectively equivalent to and can be correlated withconventional life-time stress data gathered by applying a stress voltageto the gate dielectric by contacting methods known in the art. Thiscorrelation can be derived as described below.

Conventional voltage stress techniques comprise applying a voltage, andhence an electric field, by contacting the thin gate dielectricelectrode. The stress is applied for a period of time and both theChange in Interface State Density (ΔD_(it)) and the Charge-to-Breakdown(Q_(bd)) and hence the Time-to-Breakdown (t_(bd)) are determined. FIG. 3shows the typical results for ΔD_(it) versus injected charge (Q_(inj))for three different dielectric processes. Each specific dielectricprocess affects the magnitude of degradation under a certain appliedelectric field, as shown in FIG. 3. Thus, the relationship betweenΔD_(it) and Q_(inj) can be expressed as shown in Equation 1:

ln(ΔD _(it))=[k×ln(Q _(inj))]+ln(Ap)  (1)

where k and Ap are constants, and Ap is a function of the dielectricquality and processing conditions. From Equation 1, Equations 2 and 3can be obtained:

ln(ΔD _(it))=ln[Ap×Q _(inj) ^(k)]  (2)

ΔD _(it) =Ap×Q _(inj) ^(k)  (3)

where for process 1 the value of Ap will be Ap₁, for process 2 the valueof Ap will be Ap₂, and so on.

According to the conventional stress procedure Q_(bd) is given byEquation 4: $\begin{matrix}{Q_{bd} = \frac{{qN}^{BD}\left( {T_{OX},{Temp}} \right)}{P_{gen}\left( {V,\lbrack{Process}\rbrack,{Temp}} \right)}} & (4)\end{matrix}$

where q is the electron charge and N^(BD) is the critical defect densityat breakdown and is a function of dielectric thickness T_(ox) andtemperature, but is not a function of the dielectric processingconditions.

P_(gen) is the defect generation probability, which is a function of theprocessing conditions, as well as a function of the stress voltage andtemperature. The generation probability is given by Equation 5:$\begin{matrix}{{P_{gen}\overset{\Delta}{=}{\frac{\Delta \quad D_{it}}{\Delta \quad Q_{inj}}\overset{\Delta}{=}\frac{\left( {\Delta \quad {J/J_{o}}} \right)}{\Delta \quad Q_{inj}}}},} & (5)\end{matrix}$

where ΔJ/J_(o) is the relative change in the stress-induced leakagecurrent, and ΔD_(it) is the change in interface state density as before.As shown in Equation 5, P_(gen) is process dependent. Thus, for acertain ΔJ/J_(o), a first process (Process 1) may require an injectedcharge having a value Q_(inj1) whereas a second process (Process 2) mayrequire an injected charge having a value Q_(inj2).

Dielectrics produced by Processes 1 and 2 but having the same thicknessand stressed at the same stress conditions of voltage and temperaturethus have the following relationship, using Equations 4 and 5:$\begin{matrix}{\frac{Q_{bd1}}{Q_{bd2}} = \frac{{qN}^{BD} \times P_{gen2}}{P_{gen1} \times {qN}^{BD}}} & (6) \\{\frac{Q_{bd1}}{Q_{bd2}} = {\frac{P_{gen2}}{P_{gen1}} = \frac{\left( {\Delta \quad {J/J_{o}}} \right) \times Q_{inj1}}{Q_{inj2} \times \left( {\Delta \quad {J/J_{o}}} \right)}}} & (7) \\{\frac{Q_{bd1}}{Q_{bd2}} = \frac{Q_{inj1}}{Q_{inj2}}} & (8)\end{matrix}$

where N^(BD) is the same for Processes 1 and 2, and Q_(inj1) andQ_(inj2) are the injected charges for Processes 1 and 2, respectively,required to reach the same level of degradation ΔJ/J_(o) or ΔD_(it).

For a hydrogen exposure non-contact stress system, ΔD_(it) due tohydrogen exposure can be expressed as shown in Equation 9:

ΔD _(it) =ΔD _(it) ^(x)[1−exp(−βφ_(H) ^(o) t)]  (9)

where ΔD_(it) ^(x) is the maximum number of interface states that can becreated by hydrogen dose and is independent of dielectric processingconditions. φ_(H) ^(o) is the atomic hydrogen flux into the interfacialregion, and t is the exposure time. β is a constant that is dependent ondielectric processing conditions. Equation 9 can also be written asshown in Equation 10:

ΔD _(it) =ΔD _(it) ^(x)[1−exp(−βH ^(o))]  (10)

where H^(o) is the hydrogen dose.

For hydrogen dose values well below saturation, Equation 10 can bewritten as shown in Equation 11:

ΔD _(it) =ΔD _(it) ^(x)[1−1+(βH ^(o))]

ΔD _(it) =ΔD _(it) ^(x) βH ^(o)  (11)

The relative change in interface states with respect to hydrogen dose isgiven by: $\begin{matrix}{\frac{\Delta \quad D_{it}}{H^{o}} = {\Delta \quad D_{it}^{x}\beta}} & (12)\end{matrix}$

Equation 12 represents a straight line with a slope β, where β isdependent on dielectric processing conditions, as shown in FIG. 4 forthree different processes. Thus for Processes 1 and 2, one can write:$\begin{matrix}{\frac{\Delta \quad D_{it1}}{\Delta \quad D_{it2}} = {\frac{\Delta \quad D_{it}^{x}\beta_{1}H^{o}}{\Delta \quad D_{it}^{x}\beta_{2}H^{o}} = \frac{\beta_{1}}{\beta_{2}}}} & (13)\end{matrix}$

where β₁ and β₂ are values of β for Processes 1 and 2, respectively, andΔD_(it1) and ΔD_(it2) are the corresponding degradations produced by thesame value of hydrogen dose for Processes 1 and 2, respectively.

The hydrogen non-contact stress system can be related to theconventional voltage stress system by Equation 3, producing Equations 14and 15. $\begin{matrix}{\frac{\Delta \quad D_{it1}}{\Delta \quad D_{it2}} = \frac{A_{P1}Q_{inj1}^{k}}{A_{P2}Q_{inj2}^{k}}} & (14) \\{\frac{\Delta \quad D_{it1}}{\Delta \quad D_{it2}} = \frac{A_{P1}}{A_{P2}}} & (15)\end{matrix}$

where ΔD_(it1) and ΔD_(it2) are the degradation for Processes 1 and 2,respectively, produced by the same value of injected charge, and A_(P1)and A_(P2) are the values of Ap corresponding to processes 1 and 2.Although equation 13 gives the relative degradation by hydrogen systemand equation 15 gives the relative degradation by conventional voltagestress, the degradation is measured the same way for both systems, soEquations 13 and 15 are effectively equivalent. Thus, the constants forProcess 2 can be related to those of Process 1 as shown in Equations 16and 17: $\begin{matrix}{\frac{{Ap}_{1}}{{Ap}_{2}} = \frac{\beta_{1}}{\beta_{2}}} & (16) \\{{Ap}_{2} = {{Ap}_{1} \times \frac{\beta_{2}}{\beta_{1}}}} & (17)\end{matrix}$

To find Q_(bd2) for Process 2, in Equation 14, at breakdown bothprocesses will reach the same value of ΔD_(it) (ΔD_(it1)=ΔD_(it2)), butthe ratio of the required injected charges is given by: $\begin{matrix}{\left( \frac{Q_{inj1}}{Q_{inj2}} \right)^{k} = \frac{{Ap}_{2}}{{Ap}_{1}}} & (18)\end{matrix}$

Equations 17 and 18 can be combined with equation 8 to obtain Equation19, which relates Q_(bd2) for Process 2 to Q_(bd1) for Process 1:$\begin{matrix}{{{Q_{bd2} = {{Q_{bd1} \times \frac{Q_{inj2}}{Q_{inj1}}} = {Q_{bd1} \times \left( \frac{{Ap}_{1}}{{Ap}_{2}} \right)^{1/k}}}}{Q_{bd2} = {Q_{bd1} \times \left( {\beta_{1}/\beta_{2}} \right)^{1/k}}}}\quad} & (19)\end{matrix}$

One can also write: $\begin{matrix}{t_{bd2} = {\frac{t_{bd1} \times J_{1}}{J_{2}} \times \left( {\beta_{1}/\beta_{2}} \right)^{1/k}}} & (20)\end{matrix}$

where t_(bd1) and t_(bd2) are the Time-to-Breakdown for Processes 1 and2, respectively, and J₁ and J₂ are the current densities for the twoprocesses at the desired operating conditions.

The calibration of the hydrogen stress system with the conventionalvoltage stress system is done as follows:

(A) Reference stress data is gathered by conventional contacting voltagestress procedures to determine t_(bd1), Q_(bd1), J₁, and k for adielectric produced by a reference process (Process 1). k is the slopeof the straight line relating ΔD_(it) to Q_(inj), as shown in FIG. 3.

(B) The dielectric produced by a reference Process 1 is subjected to thehydrogen non-contact stress system to determine the value of β₁ from theslope of the straight line relating ΔD_(it) to hydrogen dose H^(o) asshown in FIG. 4.

(C) A dielectric produced by a candidate new process (Process 2) isevaluated using the hydrogen non-contact stress system to determine β₂.

(D) If the dielectric produced by Process 1 has the same thickness asthe dielectric produced by Process 2, then equation 20 is used todetermine t_(bd2).

(E) If the dielectrics produced by Process 1 and 2 are of differentthicknesses, Equation 4 leads to Equation 21: $\begin{matrix}{\frac{Q_{bd1}}{Q_{bd2}} = \frac{N_{1}^{BD}Q_{inj1}}{N_{2}^{BD}Q_{inj2}}} & (21)\end{matrix}$

 where N₁ ^(BD) and N₂ ^(BD) are the thickness-dependent critical defectdensities for Processes 1 and 2, respectively, and are given in FIG. 5.Using Equations 18 and 17 with Equation 21, one obtains Equations 22 and23: $\begin{matrix}{Q_{bd2} = {{Q_{bd1}\frac{N_{2}^{BD}}{N_{1}^{BD}}} \times \left( {\beta_{1}/\beta_{2}} \right)^{1/k}}} & (22)\end{matrix}$

 t _(bd2)=(t _(bd1) ×J ₁ /J ₂)×(N ₂ ^(BD) /N ₁^(BD))×(β₁/β₂)^(1/k)  (23)

Thus, step 150 as shown in FIG. 2 may comprise comparing the rate ofchange in interface-state density of the sample wafer to a previouslydetermined rate of change in interface-state density of a referencedielectric on a reference wafer produced by a reference process, wherethe reference dielectric has a known time-to-breakdown,charge-to-breakdown, stress-induced leakage current, and associatedconstants as determined via contact voltage stress procedures.

Because lateral transport of atomic hydrogen in largemetal-oxide-semiconductor (MOS) capacitors with aluminum or polysilicongates is extremely limited, gate-free samples are preferably used in themethod of this invention. Finger-patterned gate structures with narrowfingers in the sub-micron range may also be used.

The interface-state density is preferably measured by direct waferprobing with a mercury probe using the high-low frequencycapacitance-voltage method, or alternatively, by using charge pumping oranother suitable method. Other significant dielectric parameters, suchas trapped charge and stress-induced leakage current, can also bemeasured by direct wafer probing with a mercury probe using the high-lowfrequency capacitance-voltage method.

The method described above may be used to compare at least two samplewafers, each sample wafer manufactured by a different candidatedielectric process. The dielectric process that produces the samplewafer having the highest projected charge-to-breakdown ortime-to-breakdown may then be selected as the favored process.

Although illustrated and described above with reference to certainspecific embodiments, the present invention is nevertheless not intendedto be limited to the details shown. Rather, various modifications may bemade in the details within the scope and range of equivalents of theclaims and without departing from the spirit of the invention.

What is claimed:
 1. A method for evaluating the performance of a testdielectric material for use as a gate dielectric, the method comprising:a) measuring an initial value of interface-state density in said testdielectric; b) exposing said test dielectric material to a concentrationof atomic hydrogen via a remote plasma; and c) measuring a post-exposurevalue of interface-state density in said test dielectric.
 2. The methodof claim 1 further comprising: d) repeating steps (b) and (c) at leastone more time with an incrementally higher concentration of atomichydrogen each time steps (b) and (c) are repeated; and e) determining arate of change in interface-state density value as a function of atomichydrogen concentration.
 3. The method of claim 2 further comprisingrelating the rate of change in interface-state density value toprojected charge-to-breakdown or time-to-breakdown of the testdielectric layer when said dielectric is used as the gate dielectric. 4.The method of claim 3 wherein said test dielectric is coated on a samplewafer and the relating step comprises comparing the rate of change ininterface-state density of said test dielectric coated on said samplewafer to a previously determined rate of change in interface-statedensity of a reference gate dielectric coated on a reference waferproduced by a reference process, said reference dielectric having knownvalues for time-to-breakdown (t_(bd1)), charge-to-breakdown (Q_(bd1)),current density (J₁), and slope (k) of change-in-interface-state densityversus injected charge.
 5. The method of claim 4 comprising measuringthe rate of change in interface-state density by direct wafer probingwith a mercury probe.
 6. The method of claim 5 further comprisingmeasuring the rate of change in interface-state density using a high-lowfrequency capacitance-voltage method or a charge pumping method.
 7. Themethod of claim 4 wherein the test dielectric layer coated on the samplewafer and the reference gate dielectric on said reference wafer eachhave a thickness, the thickness of the test dielectric layer on thesample wafer being equal to the thickness of the reference gatedielectric on said reference wafer, and the step of relating comprisescalculating time-to-breakdown (t_(bd2)) of the test dielectric as:$t_{bd2} = {\frac{t_{bd1} \times J_{1}}{J_{2}} \times \left( {\beta_{1}/\beta_{2}} \right)^{1/k}}$

where J₂ is a value for current density in the test dielectric, β₁ is avalue for slope of change-in-interface-state density versus hydrogendose for said reference dielectric, and β₂ is a value for slope ofchange-in-interface-state density versus hydrogen dose for said testdielectric.
 8. The method of claim 4 wherein the test dielectric layercoated on the sample wafer and the reference gate dielectric on saidreference wafer each have a thickness, the thickness of the testdielectric layer on the sample wafer being equal to the thickness of thereference gate dielectric on said reference wafer, and the step ofrelating comprises calculating charge-to-breakdown (Q_(bd2)) of the testdielectric layer on the sample wafer as: Q _(bd2) =Q_(bd1)×(β₁/β₂)^(1/k) where β₁ is a value for slope ofchange-in-interface-state density versus hydrogen dose for saidreference gate dielectric, and β₂ is a value for slope ofchange-in-interface-state density versus hydrogen dose for said testdielectric.
 9. The method of claim 4 wherein the test dielectric layercoated on the sample wafer and the reference gate dielectric on saidreference wafer each have a thickness, the thickness of the testdielectric layer on the sample wafer being unequal to the thickness ofthe reference gate dielectric on said reference wafer, and the step ofrelating comprises calculating time-to-breakdown (t_(bd2)) of the testdielectric as: t _(bd2)=(t _(bd1) ×J ₁ /J ₂)×(N ₂ ^(BD) /N ₁^(BD))×(β₁/β₂)^(1/k) where J₁ is a value for current density for thereference dielectric at a set of desired operating condition, J2 is avalue for current density for the test dielectric at a set of desiredoperating conditions, N₁ ^(BD) is a value for critical defect densityfor the reference dielectric, N₂ ^(BD) is a value for critical defectdensity for the test dielectric, β₁ is a value for slope ofchange-in-interface-state density versus hydrogen dose for saidreference dielectric, and β₂ is a value for slope ofchange-in-interface-state density versus hydrogen dose for said testdielectric.
 10. The method of claim 4 wherein the test dielectric layercoated on the sample wafer and the reference gate dielectric on saidreference wafer each have a thickness, the thickness of the testdielectric layer on the sample wafer being unequal to the thickness ofthe reference gate dielectric on said reference wafer, and the step ofrelating comprises calculating charge-to-breakdown (Q_(bd2)) of the testdielectric layer on the sample wafer as:$Q_{bd2} = {{Q_{bd1}\frac{N_{2}^{BD}}{N_{1}^{BD}}} \times \left( {\beta_{1}/\beta_{2}} \right)^{1/k}}$

where N₁ ^(BD) is a value for critical defect density for the referenceprocess, N₂ ^(BD) is a value for critical defect density for the testdielectric, β₁ is a value for slope of change-in-interface-state densityversus hydrogen dose for said reference dielectric, and β₂ is a valuefor slope of change-in-interface-state density versus hydrogen dose forsaid test dielectric.
 11. The method of claim 6 further comprisingmeasuring the rate of change in interface-state density, trapped charge,and stress-induced leakage current using said high-low frequencycapacitance-voltage method.